namespace MoreLinq;

using System;
using System.Collections.Generic;

static partial class MoreEnumerable
{
    /// <summary>
    /// Returns a sequence with each null reference or value in the source
    /// replaced with the following non-null reference or value in
    /// that sequence.
    /// </summary>
    /// <param name="source">The source sequence.</param>
    /// <typeparam name="T">Type of the elements in the source sequence.</typeparam>
    /// <returns>
    /// An <see cref="IEnumerable{T}"/> with null references or values
    /// replaced.
    /// </returns>
    /// <remarks>
    /// This method uses deferred execution semantics and streams its
    /// results. If references or values are null at the end of the
    /// sequence then they remain null.
    /// </remarks>

    public static IEnumerable<T> FillBackward<T>(this IEnumerable<T> source)
    {
            return source.FillBackward(e => e == null);
        }

    /// <summary>
    /// Returns a sequence with each missing element in the source replaced
    /// with the following non-missing element in that sequence. An
    /// additional parameter specifies a function used to determine if an
    /// element is considered missing or not.
    /// </summary>
    /// <param name="source">The source sequence.</param>
    /// <param name="predicate">The function used to determine if
    /// an element in the sequence is considered missing.</param>
    /// <typeparam name="T">Type of the elements in the source sequence.</typeparam>
    /// <returns>
    /// An <see cref="IEnumerable{T}"/> with missing values replaced.
    /// </returns>
    /// <remarks>
    /// This method uses deferred execution semantics and streams its
    /// results. If elements are missing at the end of the sequence then
    /// they remain missing.
    /// </remarks>

    public static IEnumerable<T> FillBackward<T>(this IEnumerable<T> source, Func<T, bool> predicate)
    {
            if (source == null) throw new ArgumentNullException(nameof(source));
            if (predicate == null) throw new ArgumentNullException(nameof(predicate));

            return FillBackwardImpl(source, predicate, null);
        }

    /// <summary>
    /// Returns a sequence with each missing element in the source replaced
    /// with the following non-missing element in that sequence. Additional
    /// parameters specify two functions, one used to determine if an
    /// element is considered missing or not and another to provide the
    /// replacement for the missing element.
    /// </summary>
    /// <param name="source">The source sequence.</param>
    /// <param name="predicate">The function used to determine if
    /// an element in the sequence is considered missing.</param>
    /// <param name="fillSelector">The function used to produce the element
    /// that will replace the missing one. Its first argument receives the
    /// current element considered missing while the second argument
    /// receives the next non-missing element.</param>
    /// <typeparam name="T">Type of the elements in the source sequence.</typeparam>
    /// An <see cref="IEnumerable{T}"/> with missing values replaced.
    /// <returns>
    /// An <see cref="IEnumerable{T}"/> with missing elements filled.
    /// </returns>
    /// <remarks>
    /// This method uses deferred execution semantics and streams its
    /// results. If elements are missing at the end of the sequence then
    /// they remain missing.
    /// </remarks>

    public static IEnumerable<T> FillBackward<T>(this IEnumerable<T> source, Func<T, bool> predicate, Func<T, T, T> fillSelector)
    {
            if (source == null) throw new ArgumentNullException(nameof(source));
            if (predicate == null) throw new ArgumentNullException(nameof(predicate));
            if (fillSelector == null) throw new ArgumentNullException(nameof(fillSelector));

            return FillBackwardImpl(source, predicate, fillSelector);
        }

    static IEnumerable<T> FillBackwardImpl<T>(IEnumerable<T> source, Func<T, bool> predicate, Func<T, T, T>? fillSelector)
    {
            List<T>? blanks = null;

            foreach (var item in source)
            {
                var isBlank = predicate(item);
                if (isBlank)
                {
                    (blanks ??= []).Add(item);
                }
                else
                {
                    if (blanks != null)
                    {
                        foreach (var blank in blanks)
                        {
                            yield return fillSelector != null
                                ? fillSelector(blank, item)
                                : item;
                        }

                        blanks.Clear();
                    }
                    yield return item;
                }
            }

            if (blanks?.Count > 0)
            {
                foreach (var blank in blanks)
                    yield return blank;
            }
        }
}
